Wastewater treating method and wastewater treating apparatus

ABSTRACT

There is here disclosed a wastewater treating technique for treating nitrogen compounds in a for-treatment wastewater by an electrochemical technique, wherein at least portions of a pair of electrodes is immersed in the for-treatment wastewater; a material of one electrode constituting an anode is an insoluble conductor; and a material of the other electrode constituting a cathode is an element in the group VIII of the periodic table, a conductor containing the element in the group VIII, or a conductor covered with the element in the same group or the conductor containing the element in the same group.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus for treatinga wastewater containing nitrogen compounds such as organic nitrogen,nitrite nitrogen, nitrate nitrogen and ammonia nitrogen, and phosphoruscompounds such as phosphoric acid and phosphate ions (hereinafter, “awastewater to be treated” will be referred to as “a for-treatmentwastewater”).

2. Description of the Related Art

It is already well known that the existence of nitrogen compounds is oneof causes of eutrophication of rivers and lakes. The nitrogen compoundsmuch exist in domestic life wastewater or industrial wastewater, but itis difficult to purify them and there are no effective countermeasuresup to date. In general, a biological treatment has been implemented.However, the biological treatment comprises two processes, i.e., anitrification process for converting ammonia nitrogen to nitratenitrogen, and a denitrification process for converting nitrate nitrogento a nitrogen gas. Accordingly, there has been a problem that twodifferent reaction vessels are required. There has been a furtherproblem that because a time required for the treatment is long, thetreatment efficiency is low.

Further, in this biological treatment, there has been another problemthat a large-capacity anaerobic vessel is necessary for keepingnitrifying bacteria and denitrifying bacteria, thereby to induce theincrease in equipment construction cost and apparatus installation area.There has been a further problem that the nitrifying bacteria and thedenitrifying bacteria are largely influenced by ambient temperatureenvironment, components contained in the for-treatment wastewater, andthe like, and in particular, during the winter season when thetemperature is low, their activities are lowered to deteriorate thedenitrifying action, resulting in an unstable processing efficiency.

Accordingly, there has been proposed a method for solving the foregoingtechnical problems, wherein a current is fed to the for-treatmentwastewater to decompose ammonia, nitrite nitrogen and nitrate nitrogenthrough oxidation or reduction into a nitrogen gas. In such aconventional electrolytic process for the for-treatment wastewater, anoble metal such as platinum, iridium or palladium is used as an anode.

In this process, a current is allowed to flow through the for-treatmentwastewater, so that ammonia nitrogen is oxidized with active oxygen orhypochlorous acid at the anode, and nitrogen compounds are convertedinto a nitrogen gas, whereby the treatment of the nitrogen compounds isaccomplished.

In addition to the above process, there is another process in which ironis used for electrodes constituting an anode and a cathode, and acurrent is allowed to flow through the for-treatment wastewater, so thatammonia, nitrite nitrogen and nitrate nitrogen are oxidized or reducedto decompose the for-treatment wastewater into a nitrogen gas.

However, in the conventional nitrogen compounds treating method based onthe electrolysis, a removal treatment ability of the nitrogen compoundsis poor, and hence, it is difficult to treat the nitrogen compounds inthe actual treatment of the life wastewater or the industrialwastewater. In addition, nitrate nitrogen scarcely becomes the nitrogengas, and the removal of nitrate ions at a low concentration isdifficult. Consequently, there is also a problem that they remain as anitrogen component in the wastewater and they cannot be removed.

Furthermore, in the nitrogen treating method by the electrolysis inwhich the iron electrodes are used, the nitrogen compounds in thefor-treatment wastewater are treated by use of iron dissolved in thefor-treatment wastewater. Therefore, it is necessary to dissolve a greatdeal of iron in the for-treatment wastewater, which causes a problem ofdurability. Moreover, the iron ions dissolved in the for-treatmentwastewater brings about a precipitation reaction together withphosphorus compounds present in the for-treatment wastewater, and atthis time, they are precipitated in the form of iron phosphate and thelike. In this case, a sludge is produced in large quantities from ironphosphate and the like, which causes a problem of its disposal.

Accordingly, the present applicant has already suggested a method inwhich an alloy of copper and zinc, copper and nickel, or the like isused as a cathode, and a noble metal material such as platinum, iridiumor palladium is used as an anode to electrolyze a for-treatmentwastewater. According to this treatment method, a reductive reaction ofnitrate nitrogen in the for-treatment wastewater into nitrite nitrogenand ammonia is accelerated at the cathode, and ammonia produced at thecathode can bring about a denitrification reaction together withhypochlorous acid produced at the anode. The thus obtainable synergisticeffect enables shortening a time taken for the reductive reaction, andtreating nitrate ions even at a low concentration.

However, in a case where an alloy containing copper in the cathode isused, harmful copper ions dissolved in the for-treatment wastewaterinevitably cause a problem. Therefore, it has been desired to develop atreatment method of the nitrogen compounds without any production ofharmful substances.

Furthermore, in the conventional treatment method of the nitrogencompounds by the electrolysis, a surface area of a portion of anelectrode constituting the anode which is immersed in the for-treatmentwastewater is substantially equal to that of a portion of an electrodeconstituting the cathode which is immersed in the for-treatmentwastewater. In consequence, a reductive reaction of nitrate nitrogen,i.e., nitrate ions into nitrite ions at the cathode is inhibited by anoxidation reaction of nitrite ions into nitrate ions at the anode,whereby a treatment ability of removing the nitrogen compoundsinconveniently deteriorates.

The present invention has been developed to solve the above conventionaltechnical problems, and an object of the present invention is to providea wastewater treating method and a wastewater treating apparatus bywhich nitrogen compounds are effectively removed without any productionof harmful substances, and miniaturization of the apparatus and decreasein costs can be achieved.

SUMMARY OF THE INVENTION

A first aspect of the present invention is directed to a wastewatertreating method for treating nitrogen compounds in a for-treatmentwastewater by an electrochemical technique, wherein at least portions ofa pair of electrodes are immersed in the for-treatment wastewater, amaterial of one of the electrodes constituting an anode is an insolubleconductor, and a material of the other electrode constituting a cathodeis an element in the group VIII of the periodic table, a conductorcontaining the element in the group VIII, or a conductor covered withthe element in the same group or the conductor containing the element inthe same group.

A second aspect of the present invention is directed to the abovewastewater treating method, wherein the for-treatment wastewater ispartitioned into a region of the one electrode constituting the anodeand a region of the other electrode constituting the cathode by acation-exchange membrane.

A third aspect of the present invention is directed to the abovewastewater treating method, further comprising an ammonia removaltreatment step of removing ammonia nitrogen produced in thefor-treatment wastewater from the for-treatment wastewater by the abovetreatment, and a phosphorus removal treatment step of switchingpolarities of the electrodes to treat phosphorus compounds in thefor-treatment wastewater by an electrochemical technique.

A fourth aspect of the present invention is directed to the abovewastewater treating method, wherein hypohalogenous acid is added to thefor-treatment wastewater in the ammonia removal treatment step.

A fifth aspect of the present invention is directed to the wastewatertreating method, wherein the material of the one electrode constitutingthe anode is a conductor capable of generating hypohalogenous acid,ozone or active oxygen by an electrochemical technique.

A sixth aspect of the present invention is directed to the wastewatertreating method, wherein after completion of the above treatment,polarities of the electrodes are switched, and phosphorus compounds inthe for-treatment wastewater are treated by an electrochemicaltechnique.

A seventh aspect of the present invention is directed to a wastewatertreating method for treating nitrogen compounds in a for-treatmentwastewater, wherein the for-treatment wastewater is partitioned into oneregion and the other region by a cation-exchange membrane, the methodcomprising a first treatment step in which at least a portion of a firstelectrode is immersed in the one region; at least a portion of a secondelectrode is immersed in the other region; the first electrodeconstituting an anode is a conductor; a material of the second electrodeconstituting a cathode is an element in the group VIII of the periodictable, a conductor containing the element in the group VIII, or aconductor covered with the element in the same group or the conductorcontaining the element in the same group; and the for-treatmentwastewater is treated by an electrochemical technique, and a secondtreatment step in which at least a portion of a third electrode isimmersed in the other region; a material of the third electrode is aconductor capable of generating hypohalogenous acid, ozone or activeoxygen by an electrochemical technique; the third electrode is used asthe anode; the first electrode is used as the cathode; and thefor-treatment wastewater treated by the first treatment step is treatedby the electrochemical technique.

An eighth aspect of the present invention is directed to the abovewastewater treating method, further comprising a third treatment step inwhich after completion of the second treatment step, the secondelectrode is used as the anode; the first electrode is used as thecathode; and phosphorus compounds in the for-treatment wastewater aretreated by the electrochemical technique.

A ninth aspect of the present invention is directed to the abovewastewater treating method, wherein the conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique is a noble metal, a conductor covered with the noble metal, aceramic conductor, a carbon-based conductor or a stainless steel.

A tenth aspect of the present invention is directed to the abovewastewater treating method, wherein the ceramic conductor is a ferrite.

An eleventh aspect of the present invention is directed to a wastewatertreating apparatus for treating nitrogen compounds in a for-treatmentwastewater, comprising a pair of electrodes which is at least partiallyimmersed in the for-treatment wastewater, wherein a material of one ofthe electrodes is an insoluble conductor; a material of the otherelectrode is an element in the group VIII of the periodic table, aconductor containing the element in the group VIII, or a conductorcovered with the element in the same group or the conductor containingthe element in the same group; and the one electrode is used as ananode; the other electrode is used as a cathode; and the for-treatmentwastewater is treated by an electrochemical technique.

A twelfth aspect of the present invention is directed to the abovewastewater treating apparatus, wherein the material of the one electrodeis a conductor capable of generating hypohalogenous acid, ozone oractive oxygen by an electrochemical technique.

A thirteenth aspect of the present invention is directed to the abovewastewater treating apparatus, further comprising means for switchingpolarities of the electrodes to treat phosphorus compounds in thefor-treatment wastewater by an electrochemical technique.

A fourteenth aspect of the present invention is directed to a wastewatertreating apparatus for treating nitrogen compounds in a for-treatmentwastewater, comprising a cation-exchange membrane for partitioning thefor-treatment wastewater into one region and the other region, a firstelectrode which is at least partially immersed in the one region, and asecond electrode and a third electrode which are at least partiallyimmersed in the other region, wherein a material of the first electrodeis a conductor; a material of the second electrode is an element in thegroup VIII of the periodic table, a conductor containing the element inthe group VIII, or a conductor covered with the element in the samegroup or the conductor containing the element in the same group; and amaterial of the third electrode is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by an electrochemicaltechnique; the apparatus being adapted to perform a first treatment stepin which the first electrode is used as an anode; the second electrodeis used as a cathode; and the for-treatment wastewater are treated bythe electrochemical technique, and a second treatment step in whichafter completion of the first treatment step, the third electrode isused as the anode; the first electrode is used as the cathode; and thefor-treatment wastewater is treated by the electrochemical technique.

A fifteenth aspect of the present invention is directed to the abovewastewater treating apparatus, which is adapted to perform a thirdtreatment step in which after completion of the second treatment step,the second electrode is used as the anode; the first electrode is usedas the cathode; and phosphorus compounds in the for-treatment wastewaterare treated by the electrochemical technique.

A sixteenth aspect of the present invention is directed to the abovewastewater treating apparatus, wherein the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique is a noble metal, a conductor covered with thenoble metal, a ceramic conductor, a carbon-based conductor or astainless steel.

A seventeenth aspect of the present invention is directed to the abovewastewater treating apparatus, wherein the ceramic conductor is aferrite.

An eighteenth aspect of the present invention is directed to awastewater treating method for treating nitrogen compounds in afor-treatment wastewater, wherein at least portions of a pair ofelectrodes are immersed in the for-treatment wastewater, a material ofone electrode constituting an anode is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by an electrochemicaltechnique, a material of the other electrode constituting a cathode is aconductor containing iron, and a surface area of a portion of the otherelectrode which is at least immersed in the for-treatment wastewater isset to be larger than a surface area of a portion of the one electrodewhich is at least immersed in the for-treatment wastewater; and thefor-treatment wastewater is treated by the electrochemical technique.

A nineteenth aspect of the present invention is directed to the abovewastewater treating method, wherein after completion of the abovetreatment, polarities of the electrodes are switched; and phosphoruscompounds in the for-treatment wastewater are treated by anelectrochemical technique.

A twentieth aspect of the present invention is directed to a wastewatertreating method for treating nitrogen compounds in a for-treatmentwastewater, comprising a first treatment step in which at least portionsof first and second electrodes are immersed in the for-treatmentwastewater; a material of the first electrode constituting an anode is aconductor; a material of the second electrode constituting a cathode isa conductor containing iron; a surface area of a portion of the secondelectrode which is at least immersed in the for-treatment wastewater isset to be larger than a surface area of a portion of the first electrodewhich is at least immersed in the for-treatment wastewater; and thefor-treatment wastewater is treated by the electrochemical technique; asecond treatment step in which at least a portion of a third electrodeis immersed in the for-treatment wastewater; a material of the thirdelectrode is a conductor capable of generating hypohalogenous acid,ozone or active oxygen by an electrochemical technique, when the thirdelectrode is used as an anode; a surface area of a portion of the thirdelectrode which is at least immersed in the for-treatment wastewater isset to be larger than a surface area of a portion of the first electrodewhich is at least immersed in the for-treatment wastewater; and aftercompletion of the first treatment step, the third electrode is used asthe anode; the second electrode is used as the cathode; and thefor-treatment wastewater are treated by the electrochemical technique.

A twenty-first aspect of the present invention is directed to the abovewastewater treating method, wherein an operation transfers from thefirst treatment step to the second treatment step, when ammonia andammonium-ions in the for-treatment wastewater have reached predeterminedconcentrations.

A twenty-second aspect of the present invention is directed to the abovewastewater treating method, further comprising a third treatment step inwhich after completion of the second treatment step, the third electrodeis used as the cathode; the second electrode is used as the anode; andphosphorus compounds in the for-treatment wastewater are treated by theelectrochemical technique.

A twenty-third aspect of the present invention is directed to the abovewastewater treating method, wherein a stainless steel is used as thematerial of the other electrode or the second electrode.

A twenty-fourth aspect of the present invention is directed to the abovewastewater treating method, wherein as the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique, there is used a noble metal or a conductorcovered with the noble metal.

A twenty-fifth aspect of the present invention is directed to the abovewastewater treating method, wherein as the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique, there is used a ceramic conductor or aconductor covered with the ceramic conductor.

A twenty-sixth aspect of the present invention is directed to the abovewastewater treating method, wherein as the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique, there is used a carbon-based conductor or aconductor covered with the carbon-based conductor.

A twenty-seventh aspect of the present invention is directed to theabove wastewater treating method, wherein as the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique, there is used a stainless steel or aconductor covered with the stainless steel.

A twenty-eighth aspect of the present invention is directed to awastewater treating apparatus for treating nitrogen compounds in afor-treatment wastewater, comprising a pair of electrodes which is atleast partially immersed in the for-treatment wastewater, wherein amaterial of one of the electrodes is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by an electrochemicaltechnique; a material of the other electrode is a conductor-containingiron; and a surface area of a portion of the other electrode which is atleast immersed in the for-treatment wastewater is set to be larger thana surface area of a portion of the one electrode which is at leastimmersed in the for-treatment wastewater, and the one electrode is usedas an anode; the other electrode is used as a cathode; and thefor-treatment wastewater is treated by the electrochemical technique.

A twenty-ninth aspect of the present invention is directed to the abovewastewater treating apparatus, further comprising means for switchingpolarities of the electrodes to treat phosphorus compounds in thefor-treatment wastewater by an electrochemical technique.

A thirtieth aspect of the present invention is directed to a wastewatertreating apparatus for treating nitrogen compounds in a for-treatmentwastewater, comprising first, second and third electrodes which are atleast partially immersed in the for-treatment wastewater, wherein thefirst electrode is a conductor; a material of the second electrode is aconductor containing iron; a material of the third electrode is aconductor capable of generating hypohalogenous acid, ozone or activeoxygen by an electrochemical technique when the third electrode is usedas an anode; and a surface area of portions of the second and thirdelectrodes which are at least immersed in the for-treatment wastewateris set to be larger than a surface area of a portion of the firstelectrode which is at least immersed in the for-treatment wastewater,the apparatus being adapted to perform a first treatment step in whichthe first electrode is used as an anode; the second electrode is used asa cathode; and the for-treatment waste water is treated by theelectrochemical technique; and a second treatment step in which aftercompletion of the first treatment step, the third electrode is used asthe anode; the second electrode is used as the cathode; and thefor-treatment wastewater is treated by the electrochemical technique.

A thirty-first aspect of the present invention is directed to the abovewastewater treating apparatus, further comprising means for detectingammonia and ammonium ions in the for-treatment wastewater, wherein whenammonia and ammonium ions in the for-treatment wastewater have reachedpredetermined concentrations, an operation transfers from the firsttreatment step to the second treatment step.

A thirty-second aspect of the present invention is directed to the abovewastewater treating apparatus, which is adapted to perform a thirdtreatment step in which after completion of the second treatment step,the third electrode is used as the cathode; the second electrode is usedas the anode; and phosphorus compounds in the for-treatment wastewaterare treated by the electrochemical technique.

A thirty-third aspect of the present invention is directed to the abovewastewater treating apparatus, wherein a material of the other electrodeor the second electrode is a stainless steel.

A thirty-fourth aspect of the present-invention is directed to the abovewastewater treating apparatus, wherein the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique is a noble metal or a conductor covered withthe noble metal.

A thirty-fifth aspect of the present invention is directed to the abovewastewater treating apparatus, wherein the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique is a ceramic conductor or a conductor coveredwith the ceramic conductor.

A thirty-sixth aspect of the present invention is directed to the abovewastewater treating apparatus, wherein the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique is a carbon-based conductor or a conductorcovered with the carbon-based conductor.

A thirty-seventh aspect of the present invention is directed to theabove wastewater treating apparatus, wherein the conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by theelectrochemical technique is a stainless steel or a conductor coveredwith the stainless steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing an outline of a first embodimentof a wastewater treating apparatus for realizing a wastewater treatingmethod of the present invention;

FIG. 2 is a view showing changes in concentrations of nitric acid ions(NO₃ ⁻) to be treated by the apparatus of FIG. 1;

FIG. 3 is an explanatory view showing an outline of a second embodimentof a wastewater treating apparatus of the present invention;

FIG. 4 is a view showing changes in concentrations of nitric acid ions,nitrous acid ions (NO₂ ⁻), ammonium ions (NH₄ ⁺) and phosphoric acidions (PO₄ ³⁻) to be treated by the apparatus of FIG. 3;

FIG. 5 is an explanatory view showing an outline of another example ofthe first embodiment of the wastewater treating apparatus of the presentinvention;

FIG. 6 is an explanatory view showing an outline of a third embodimentof a wastewater treating apparatus for realizing a wastewater treatingmethod of the present invention;

FIG. 7 is a view showing changes in concentrations of nitric acid ions(NO₃ ⁻) to be treated by the apparatus of FIG. 6;

FIG. 8 is an explanatory view showing an outline of a fourth embodimentof a wastewater treating apparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, preferred embodiments of the present invention will be describedwith reference to the accompanying drawings.

(1) First Embodiment

First, an embodiment of the present invention will be described byreferring to FIGS. 1 and 2. FIG. 1 is an explanatory view showing anoutline of a wastewater treating apparatus 1 for realizing thewastewater treating method of the first embodiment of the presentinvention. The wastewater treating apparatus 1 of the embodimentcomprises a treating vessel 2 which constitutes a treating chamber 4having an inlet and an outlet (not shown) of a wastewater, a pair ofelectrodes 5, 6 oppositely arranged to be partially immersed in thefor-treatment wastewater of the treating chamber 4, a power supply (notshown) to energize the electrodes 5, 6, a controller to control thepower supply, etc. Agitation means may be disposed in the treatingvessel 2 to agitate the contents therein.

The electrode 6 is made of, for example, iron (Fe) or a conductorcovered with the iron as an element in the group VIII of a periodictable, or a conductor containing the element in the group VIII, or aconductor covered with the element in the same group or a conductorcontaining the element in the same group. The electrode 5 is, forexample, a noble metal electrode made of platinum (Pt) or a mixture ofplatinum and iridium (Ir), or made of an insoluble conductor coveredwith such en element. According to the embodiment, the iron (Fe) is usedas the element in the group VIII of the periodic table, or the conductorcontaining the element in the group VIII, or the conductor covered withthe element in the same group or the conductor containing the element inthe same group. In addition, cobalt (Co), nickel (Ni) or the like may beused as long as it is a conductor containing the element in the groupVIII. The electrode 5 is the noble metal electrode, or made of theinsoluble conductor covered with such an element. In addition, aceramic-based conductor such as a ferrite, a carbon-based conductor, astainless steel or the like may be used. According to the embodiment,the mixture of platinum and iridium (platinum/iridium) is used.

Thus, the for-treatment wastewater which contains nitric acid ions asnitrate nitrogen and phosphoric acid ions as phosphorus compounds isreserved in the treating chamber 4 of the treating vessel 2. The powersupply is turned ON by the controller to apply a positive potential tothe electrode 5 and a negative potential to the electrode 6 (nitrogentreatment step, left side of FIG. 1). Accordingly, the electrode 5becomes an anode while the electrode 6 becomes a cathode.

As a result of the application of the potentials, electrons generated onthe electrode 5 side constituting the anode are fed to the electrode 6side constituting the cathode, and the nitric acid ions contained as thenitrate nitrogen in the for-treatment wastewater are reduced to nitrousacid ions (reaction A). Further, electrons are fed to the electrode 6side constituting the cathode to reduce the nitrate nitrogen reduced tothe nitrous-acid ions to ammonia (ammonium ions) (reaction B). Thereactions A and B are shown below.NO₃ ⁻+H₂O+2e ⁻→NO₂ ⁻+2OH⁻  Reaction ANO₂ ⁻+5H₂O+6e ⁻→NH₃(aq)+7OH⁻  Reaction B

On the other hand, on the platinum/iridium electrode 5 constituting theanode, chloride ions as halide ions contained in the for-treatmentwastewater emit electrons to generate chlorine. The chlorine isdissolved in the water to generate hypochlorous acid simultaneously,ozone or active oxygen is generated. According to the embodiment, thechloride ions are contained in the for-treatment wastewater to generatethe hypochlorous acid. In addition, if other halide ions are containedin the for-treatment wastewater, similar effects can be obtained evenwhen other hypohalogeneous acid such as hypofluorous acid andhypobromous acid are generated.

In the case of the low concentrations of chloride ions contained in thefor-treatment wastewater, for example, halide ions such as chlorideions, iodide ions or bromide ions, or compounds containing such halideions, e.g., a sodium chloride or a potassium chloride, may be added tothe for-treatment wastewater. That is, the chloride ions of the sodiumchloride in the for-treatment wastewater are set to 10 ppm or higher to40000 ppm or lower.

The chloride ions originally contained in the for-treatment wastewateror the sodium chloride added as described above is oxidized on theelectrode 5 constituting the anode to generate chlorine (reaction C,shown in the case of the sodium chloride). The generated chlorine reactswith water in the for-treatment wastewater to generate hypochlorous acid(reaction D). Then, the generated hypochlorous acid reacts with theammonia (ammonium ions) generated in the for-treatment wastewater in thereaction B to make a plurality of chemical changes, and then convertedinto nitrogen gas (reaction E).

The reactions C to E are shown below.NaCl→Na⁺+Cl⁻2Cl⁻→Cl₂+2e ⁻  Reaction CCl₂+H₂O→HClO+HCl  Reaction DNH₃+HClO→NH₂Cl+H₂O NH₂Cl+HClO→NHCl₂+H₂O NH₂Cl+NHCl₂→N₂⇑+3HCl  Reaction E

The ammonia (ammonium ions) in the for-treatment wastewater reacts withthe ozone or active oxygen generated on the electrode 5 sideconstituting the anode as shown in a reaction F to be denitrified tonitrogen gas.2NH₃(aq)+3(O)→N₂⇑+3H₂O  Reaction F

Accordingly, the nitrogen compounds of nitrate nitrogen, nitritenitrogen and ammonia nitrogen in the for-treatment wastewater can betreated in the same treating vessel 2.

The result of an experiment of FIG. 2 shows the concentration of nitratenitrogen in the for-treatment wastewater when a platinum, iridium-basedelectrode is used for the electrode 5 constituting the anode while theelectrode 6 constituting the cathode is made of iron or brass (alloy ofzinc and copper). In the experiment, constant-current electrolysis wascarried out by filling the beaker of 500 ml at KC 1500 ml of 100 mM, andsupplying 0.5 A between the electrodes 5, 6 for five hours.

In FIG. 2, a white circle indicates the result of measuring theconcentration of nitrate nitrogen (NO3−) for every one hour when brassis used as a cathode, and a black square indicates the same when iron isused as a cathode. It is apparent from the drawing that the use of theion electrode as the cathode exhibits treatment characteristics similarto those in the case of the brass (iron is better at a lowconcentration), and the concentration of the nitrate nitrogen becomesequal to/lower than 100 ppm of a wastewater quality standard after thepassage of one hour. Here, the elution of toxic copper ions becomes aproblem when the brass is used for the cathode. However, according tothe present invention, since the iron is used for the cathode, theproblem caused by toxicity can be solved while an efficient nitrogencompound treating ability roughly similar to that in the case of thebrass is maintained.

After the end of the nitrogen compound treatment (nitrogen treatmentstep), the controller switches the polarities of potentials applied tothe electrodes 5, 6 (right side of FIG. 1, phosphorus treatment step,the nitrogen treatment reaction continues in the for-treatmentwastewater even after the switching of the polarities). Thus, theelectrode 5 constitutes a cathode while the electrode 6 constitutes ananode. Therefore, since the for-treatment wastewater is subjected toelectrolysis as an electrochemical technique, and the electrode 6constituting the anode is made of the conductor as described above, iron(II) ions are eluted from the electrode 6 into the for-treatmentwastewater to be oxidized to iron (III) ions therein.

The generated iron (III) ions are flocculated to settle with thephosphoric acid ions in the for-treatment wastewater by adephosphorization reaction as shown in a reaction G, whereby ironphosphate which has exsolution in water is generated.Fe³⁺+PO₄ ³⁻→FePO₄↓  Reaction G

Thus, the phosphoric acid ions as phosphorus compounds contained in thefor-treatment wastewater can be flocculated to settle as iron phosphate.

A portion of the ion (III) ions eluted in the state of the iron (II)ions in the for-treatment wastewater to supply electrons, and oxidizedon the electrode or in the for-treatment wastewater receive electronsagain on the electrode 5 side constituting the cathode in this case tobe reduced to iron (II) ions, and oxidized again on the electrode 6 sideconstituting the anode or in the for-treatment wastewater.

The technology for electrolyzing the for-treatment wastewater containingthe iron (II) ions to reduce the nitric acid ions contained in thefor-treatment wastewater to the ammonium ions is disclosed in the“Development of Inorganic Nitrogen Compound Treating Technology UsingElectrochemical Reaction” of annual meeting lecture papers of JapanSociety on Water Environment distributed at the 33rd meeting of JapanSociety on Water Environment held from Mar. 16 to 18 in 1999.

Scales (CaCO₃, Mg(OH)₂ etc.) grown on the surface of the electrode 6constituting the cathode in the nitrogen treatment step are cleaned offfrom the surface when the electrode 6 becomes an anode in the phosphorustreatment step. Accordingly, the electrolytic performance of theelectrode 6 can be maintained high.

According to the embodiment, the electrode 5 is the noble metalelectrode made of platinum (Pt) or the mixture or platinum and iridium,or an insoluble conductor covered with such an element. In addition,however, the electrode 5 may be made of a ceramic-based conductor suchas a ferrite, a carbon-based conductor or a stainless steel. Further, aslong as an insoluble conductor is used, the electrode 5 may be anelectrode in which no hypohalogenous acid, ozone or active oxygen isgenerated, or the amount of generation is small by the electrochemicaltechnology. This case will be described by referring to FIG. 5.

FIG. 5 is an explanatory-view showing an outline of the wastewatertreating apparatus 1 of the other example of the first embodiment. InFIG. 5, reference numerals similar to those of FIG. 1 denote identicalor similar operations. That is, the for-treatment wastewater whichcontains nitric acid ions as nitrate nitrogen and phosphoric acid ions;as phosphorus compounds is reserved in the treating chamber 4 of thetreating vessel 2. The power supply is turned ON by the controller toapply a positive potential to the electrode 5 and a negative potentialto the electrode 6 (nitrogen treatment step, left side of FIG. 5).Accordingly, the electrode 5, becomes an anode while the electrode 6becomes a cathode.

As a result of the application of the potentials, electrons generated onthe electrode 5 side constituting the anode are fed to the electrode 6side constituting the cathode, and the nitric acid ions contained as thenitrate nitrogen in the for-treatment wastewater are reduced to nitrousacid ions (reaction A). Further, electrons are fed to the electrode 6side constituting the cathode to reduce the nitrate nitrogen reduced tothe nitrous acid ions to ammonia (ammonium ions) (reaction B).

In this case, if the electrode 5 constituting the anode is an electrodein which it is difficult to generate hypohalogeneous acid, ozone oractive oxygen by the electrochemical reaction technology as describedabove, as means for turning OFF the power supply by the controller toremove ammonia nitrogen (ammonia or ammonium ions) from thefor-treatment wastewater when the nitric acid ions as the nitratenitrogen and the nitrous acid ions as nitrite nitrogen in thefor-treatment wastewater are nearly reduced to the ammonia nitrogen(ammonia or ammonium ions), for example, a substance which has theeffect of removing nitrogen such as drug hypohalogeneous acid,electrolytic hypohalogeneous acid, ozone or the like is added (ammoniaremoval treatment step, upper center of FIG. 5).

Accordingly, the ammonia nitrogen generated or left in the for-treatmentwastewater is converted into nitrogen gas by the hypohalogeneous acid orthe ozone (the reactions E and F).

Thus, the nitrogen compounds of nitrate nitrogen, nitrite nitrogen,ammonia nitrogen etc. in the for-treatment wastewater can be treated inthe same treating vessel 2.

After the end of the removal treatment of the ammonia nitrogen from thefor-treatment wastewater (ammonia removal treatment step), thecontroller applies a positive potential to the electrode 6 and anegative potential to the electrode 6 (upper right side of FIG. 5,phosphorus treatment step, the nitrogen and ammonia treatment reactioncontinues in the for-treatment wastewater even after the application ofthe potentials). Thus, the electrode 5 constitutes a cathode while theelectrode 6 constitutes an anode. Therefore, since the for-treatmentwastewater is subjected to electrolysis as an electrochemical technique,and the electrode 6 constituting the anode is made of the conductor asdescribed above, iron (II) ions are eluted from the electrode 6 into thefor-treatment wastewater to be oxidized to iron (III) ions therein.

The generated iron (III) ions are flocculated to settle with thephosphoric acid ions in the for-treatment wastewater by adephosphorization reaction (the reaction G), whereby iron phosphatewhich has exsolution in water is generated. Thus, the phosphoric acidions as phosphorus compounds contained in the for-treatment wastewatercan be flocculated to settle as iron phosphate.

In this case, a portion of the ion (III) ions eluted in the state of theiron (II) ions in the for-treatment wastewater to supply electrons, andoxidized on the electrode or in the for-treatment wastewater receiveelectrons again on the electrode 5 side constituting the cathode in thiscase to be reduced to iron (II) ions, and oxidized again on theelectrode 6 side constituting the anode or in the for-treatmentwastewater.

According to the embodiment, after the treatment (nitrogen treatmentstep) of the nitrogen compounds in the for-treatment wastewater, thetreatment (ammonia removal treatment step) of removing the ammonianitrogen generated in the nitrogen treatment step is carried out, andthen the treatment (phosphorus treatment step) of the phosphoruscompounds in the for-treatment wastewater is carried out. In addition,however, as shown in the lower side of FIG. 5, the polarities of theelectrodes 5, 6 may be switched to treat the phosphorus compounds(phosphorus treatment step) after the end of the nitrogen treatmentstep, and then the ammonia nitrogen present in the for-treatmentwastewater may be removed (ammonia removal treatment step). Effectssimilar to those of the above case can be obtained.

Furthermore, according to the embodiment, in the ammonia removaltreatment step, the substance which has the effect of removing nitrogensuch as the drug hypohalogeneous acid, the electrolytic hypohalogeneousacid, the ozone or the like is added to remove the ammonia nitrogen. Inaddition, however, a so-called ammonia stripping technology known as amethod for removing ammonia nitrogen from a solution may be employed toremove the ammonia nitrogen in the for-treatment wastewater.

Thus, according to the embodiment, since the electrode 5 is made of theinsoluble conductor, the elution of the electrode 5 itself can beprevented to improve the durability of the electrode 5. Additionally,since the great amount of sludge is not generated by the great amount ofiron eluted in the for-treatment wastewater different from theconventional case, the workability of waste disposal maintenance andadaptability to the environment can be improved.

According to the embodiment, in the nitrogen treatment step, theelectrode of high oxygen generation efficiency and low hypohalogeneousacid generation efficiency can be used for the electrode 5 constitutingthe anode, and the increase of pH in the for-treatment wastewater whichoccurs during the reduction of the nitrate nitrogen can be suppressed.As a result, it is possible to greatly suppress a reduction in thereduction ability of the nitrate nitrogen.

For the partition of the electrodes 5, 6 of the embodiment,specifically, a later-described cation-exchange membrane (described withreference to a second embodiment) may be used. Thus, it is possible toprevent an inconvenience that nitrous acid ions generated on theelectrode 6 constituting the cathode transfer to the electrode 5 sideconstituting the anode to be oxidized. As a result, the efficiency ofthe reduction reaction of the nitrate nitrogen in the for-treatmentwastewater to the nitrite nitrogen and the ammonia can be furtherincreased.

(2) Second Embodiment

Next, another embodiment of the present invention will be described byreferring to FIGS. 3 and 4. FIG. 3 is an explanatory view showing anoutline of a wastewater treating apparatus 1 for realizing thewastewater treating method of the second embodiment of the presentinvention. In the drawings, reference numerals similar to those of FIGS.1 and 2 denote identical or similar operations.

The wastewater treating apparatus 1 of the embodiment comprises atreating vessel 2 which constitutes a treating chamber 4 having an inletand an outlet (not shown) for the wastewater therein, a pair ofelectrodes 5 (first electrode), 6 (second electrode) oppositely arrangedto be partially immersed in the for-treatment wastewater of the treatingchamber 4, a cation-exchange membrane (barrier membrane: Nafion (productname) by DUPONT) 8 disposed between the electrodes 5, 6 to partition theinside of the treating chamber 4 into one region 4A in which theelectrode 5 is present and the other region 4B in which the electrode 6is present, an electrode 7 (third electrode) arranged to be immersed atleast partially in the for-treatment wastewater of the other region 4Bof the cation-exchange membrane 8, an unshown power supply to energizethe electrodes 5, 6, 7, a controller to control the power supply, etc.Agitation means may be disposed in the treating vessel 2 to agitate thecontents therein.

The electrode 6 is made of, for example, iron (Fe) or a conductorcovered with the iron as an element in the group VIII of a periodictable, or a conductor containing the element in the group VIII, or aconductor covered with the element in the same group or a conductorcontaining the element in the same group. Each of the electrodes 5 and 7is, for example, a noble metal electrode made of platinum (Pt) or amixture of platinum and iridium (Ir), or made of an insoluble conductorcovered with such an element. According to the embodiment, the iron (Fe)is used as the element in the group VIII of the periodic table. Inaddition, cobalt (Co), nickel (Ni) or the like may be used as long as itis a conductor containing the element in the group VIII. The electrode 5is the noble metal electrode, or made of the insoluble conductor coveredwith such an element. In addition, a ceramic-based conductor such as aferrite, a carbon-based conductor, a stainless steel or the like may beused. According to the embodiment, the mixture of platinum and iridium(platinum/iridium) is used.

Thus, the for-treatment wastewater which contains nitric acid ions asnitrate nitrogen and phosphoric acid ions as phosphorus compounds isreserved in the treating chamber 4 of the treating vessel 2. The powersupply is turned ON by the controller to first apply a positivepotential to the electrode 5 and a negative potential to the electrode 6(first treatment step, left side of FIG. 3). At this time, no potentialis applied to the electrode 7. Accordingly, the electrode 5 becomes ananode while the electrode 6 becomes a cathode.

As a result of the application of the potentials, electrons generated onthe electrode 5 side constituting the anode are fed to the electrode 6side constituting the cathode, and the nitric acid ions contained as thenitrate nitrogen in the for-treatment wastewater are reduced to nitrousacid ions (the reaction A). Further, electrons are fed to the electrode6 side constituting the cathode to reduce the nitrate nitrogen reducedto the nitrous acid ions to ammonia (ammonium ions) (the reaction B).

In this case, if the nitrous acid ions generated by reducing the nitricacid ions on the electrode 6 side (other region 4B) constituting thecathode transfer to the one region 4A, an oxidation reaction occurs onthe electrode 5 side constituting the anode to oxidize the nitrous acidions to nitric acid ions (reaction H).NO₂ ⁻+H₂O→NO₃ ⁻+2H⁺+2e ⁻  Reaction H

If such an oxidation reaction occurs, the oxidation reaction of thenitrous acid ions occurs on the electrode 5 with respect to thereduction reaction of the nitric acid ions on the electrode 6 to lowernitrogen treatment efficiency. According to the embodiment, however,since the cation-exchange membrane 8 is present between the electrode 5and the electrode 6, the transfer of the nitrous acid ions to the oneregion 4 a is prevented to inhibit the reaction H. As a result, thenitrogen treatment ability to reduce the nitric acid ions to the ammonia(ammonium ions) can be improved.

After the end of the first treatment step, the controller applies anegative potential to the electrode 5 and a positive electrode to theelectrode 7 (second treatment step, center of FIG. 3). At this time, nopotential is applied to the electrode 6, or a very small negativepotential is applied to prevent iron corrosion. Accordingly, theelectrode 7 becomes an anode while the electrode 5 becomes a cathode.

In this case, on the platinum/iridium electrode 7 side constituting theanode, chloride ions as halide ions contained in the for-treatmentwastewater emit electrons to generate chlorine. The chlorine isdissolved in the water to generate hypochlorous acid (the reactions C toD). Simultaneously, ozone or active oxygen is generated. According tothe embodiment, the chloride ions are contained in the for-treatmentwastewater to generate the hypochlorous acid. In addition, if otherhalide ions are contained in the for treatment wastewater, similareffects can be obtained even when other hypohalogeneous acid such ashypofluorous acid and hypobromous acid are generated.

The generated hypochlorous acid reacts with the ammonia (ammonium ions)generated in the for-treatment wastewater in the reaction B of the firsttreatment step to make a plurality of chemical changes, and thenconverted into nitrogen gas (reaction E). The ammonia (ammonium ions) inthe for-treatment wastewater reacts with the ozone or active oxygengenerated on the electrode 7 side constituting the anode as shown in thereaction F to be denitrified to nitrogen gas.

Accordingly, the nitrogen compounds of nitrate nitrogen, nitritenitrogen and ammonia nitrogen in the for-treatment wastewater can betreated in the same treating vessel 2.

The results of experiments of the left upper side, the left lower sideand the right upper side of FIG. 4 show the concentrations of nitricacid ions (NO₃ ⁻), nitrous acid ions (NO₂ ⁻) and ammonium ions (NH₄ ⁺)in the for-treatment wastewater when platinum, iridium-based electrodesare used for the electrodes 5, 7 while the electrode 6 is made of iron.In the experiment, constant-current electrolysis was carried out byfilling the beaker of 500 ml at KC 1500 ml of 100 mM, and supplying 0.5A between the electrode 5 (anode) and the electrode 6 (cathode) for 4.3hours, and subsequently electrolysis was carried out by supplying acurrent between the electrode 5 (cathode) and the electrode 7 (anode)for 1 hour.

Each of FIG. 4 shows the result of measuring a concentration for everyone hour. It is apparent that even if the ion electrode is used as thecathode, the concentration of nitric acid ions is lowered to 50 ppm orlower one hour after the start of the experiment. So it can beunderstood that the nitrogen treatment is done more efficiently thanaforementioned embodiment. Since there is no elution of copper ions asin the aforementioned case, a problem caused by toxicity can be solvedwhile a high nitrogen compound treating ability is maintained.

On the right upper side of FIG. 4, the lowered concentration of theammonium ions is seen before the start of the second treatment stepabout two hours or thereafter from the start of the experiment. Thisoccurs because since the cation-exchange membrane 8 is present, pHbecomes high in the other region 4B to cause stripping (diffusion intogas).

After the end of the nitrogen compound treatment (first and secondnitrogen treatment steps), the controller applies a negative potentialto the electrode 5 and a positive potential to the electrode 6. At thistime, no potential is applied to the electrode 7. Alternatively, a verysmall potential is applied to prevent iron corrosion. (right side ofFIG. 3, third treatment step, the nitrogen treatment reaction continuesin the for-treatment wastewater even after the switching to thepotential application to the electrode 6). Thus, the electrode 5:constitutes a cathode while the electrode 6 constitutes an anode.Therefore, since the for-treatment wastewater is subjected toelectrolysis as an electrochemical technique, and the electrode 6constituting the anode is made of the conductor containing iron as theconductor containing the element in the group VIII of the periodic tableas described above, iron (II) ions are eluted from the electrode 6 intothe for-treatment wastewater to be oxidized to iron (III) ions therein.

The generated iron (III) ions are flocculated to settle with thephosphoric acid ions in the for-treatment wastewater by adephosphorization reaction as shown in the reaction G, whereby ironphosphate which has exsolution in water is generated. Thus, thephosphoric acid ions as phosphorus compounds contained in thefor-treatment wastewater can be flocculated to settle as iron phosphate.

The result of the experiment of the right lower side of FIG. 4 shows theconcentration of phosphoric acid ions (PO₄ ³⁻) in the for-treatmentwastewater when a platinum, iridium-based electrode is used for theelectrode 5 while the electrode 6 is made of iron. This experiment showsthe result of carrying out electrolysis for four hours by setting theelectrode 5 as a cathode and the electrode 6 as an anode and supplying acurrent after the end of the second treatment step.

This case shows the result of measuring the concentration for every onehour. It is apparent that the concentration of phosphoric acid ionswhich is initially 60 ppm is lowered by 10% or lower to about 5 ppmafter the passage of about one hour, and then it is gradually lowered toroughly approach zero after four hours. This situation is similar tothat in the phosphorus treatment step of the first embodiment.

Thus, in the third treatment step, as in the previous case, electronsgenerated on the electrode 6 side constituting the anode are fed to theelectrode 5 side constituting the cathode, and the nitric acid ionscontained as the nitrate nitrogen in the for-treatment wastewater arereduced to nitrous acid ions (reaction A). Further, electrons are fed tothe electrode 5 side constituting the cathode to reduce the nitratenitrogen reduced to the nitrous acid ions to ammonia (ammonium ions)(reaction B).

If only the nitrogen treatment step is carried out in the firstembodiment while only the first and second treatment steps are carriedout in the second embodiment, the electrode 6 may be made of a stainlesssteel. In this case, as the stainless steel, there are available anaustenite-based steel type (SUS304), an austenite/ferrite-based steeltype (SUS329J1), a ferrite-based steel type (SUS444), a martensite-basedsteel type (SUS410), a precipitation hardened steel type (SUS631) etc.The aforementioned effects of the nitrogen compound treatment can beexpected when such a stainless steel is used. In this case, especially,durability can be improved by preventing the generation of rust on theelectrodes.

According to each of the foregoing embodiments, apparently, the nitrogencompounds and the phosphorus compounds in the for-treatment wastewatercan be effectively treated by the electrochemical technique. Thus, thenitrogen compounds and the phosphorus compounds in the for-treatmentwastewater can be treated in the same treating vessel to eliminate theconventional necessity of installing a large biological treating vessel,and it is possible to prevent increases in facility construction costand apparatus installation area.

Furthermore, it is possible to eliminate the necessity of complexmaintenance work of denitrification bacteria which is necessary in thebiological treatment, and to provide stable and high nitrogen andphosphorus treating efficiency.

(3) Third Embodiment

Next, another embodiment of the present invention will be described byreferring to FIGS. 6 and 7. FIG. 6 is an explanatory view showing anoutline of a wastewater treating apparatus 1 for realizing thewastewater treating method of the third embodiment of the presentinvention. The wastewater treating apparatus 1 of the embodimentcomprises a treating vessel 2 which constitutes a treating chamber 4having an inlet and an outlet (not shown) for the wastewater therein, apair of electrodes 5, 6 oppositely arranged to be partially immersed inthe for-treatment wastewater of the treating chamber 4, an unshown powersupply to energize the electrodes 5, 6, a controller to control thepower supply, etc. Agitation means may be disposed in the treatingvessel 2 to agitate the contents therein.

The electrode 6 is made of, for example, iron (Fe) or a conductorcovered with the iron (conductor containing iron). When it constitutesan anode, the electrode 5 is, for example, a noble metal electrode madeof platinum (Pt) or a mixture of platinum and iridium (Ir), or made ofan insoluble conductor covered with such en element as a conductorcapable of generating hypohalogeneous acid such as hypochlorous acid,ozone, or active oxygen. In addition, as the conductor capable ofgenerating hypohalogeneous acid such as hypochlorous acid, ozone oractive oxygen, for example, a ceramic-based conductor, a stainlesssteel, an insoluble conductor covered with the stainless steel, acarbon-based conductor, or an insoluble conductor covered with thecarbon-based conductor may be used. In this case, the conductor can beconstructed at low cost. Especially, durability can be improved when theceramic-based conductor or the conductor such as a stainless steel isused. According to the embodiment, platinum is used.

According to the embodiment, the electrode 6 is made of a plate memberwhile the electrode 5 is made of a linear member such as a wire so thata surface area of at least a portion of the electrode 6 immersed in thefor-treatment wastewater can be larger than that of at least a portionof the electrode 5 immersed in the for-treatment wastewater.

Thus, the for-treatment wastewater which contains nitric acid ions asnitrate nitrogen and phosphoric acid ions as phosphorus compounds isreserved in the treating chamber 4 of the treating vessel 2. The powersupply is turned ON by the controller to apply a positive potential tothe electrode 5 and a negative potential to the electrode 6 (nitrogentreatment step, left side of FIG. 6). Accordingly, the electrode 5becomes an anode while the electrode 6 becomes a cathode.

As a result of the application of the potentials, electrons generated onthe electrode 5 side constituting the anode are fed to the electrode 6side constituting the cathode, and the nitric acid ions contained as thenitrate nitrogen in the for-treatment wastewater are reduced to nitrousacid ions (reaction I). Further, electrons are fed to the electrode 6side constituting the cathode to reduce the nitrate nitrogen reduced tothe nitrous acid ions to ammonia (ammonium ions) (reaction J). Thereactions I and J are shown below.NO₃ ⁻+H₂O+2e ⁻→NO₂ ⁻+2OH⁻  Reaction INO₂ ⁻+5H₂O+6e ⁻→NH₃(aq)+7OH⁻  Reaction J

In this case, when nitrous acid ions generated by reducing the nitricacid ions on the electrode 6 constituting the cathode transfer to theelectrode 5 side constituting the anode, an oxidation reaction occurs onthe electrode 5 constituting the anode to oxidize the nitrous acid ionsto nitric acid ions (reaction K).NO₂ ⁻+H₂O→NO₃ ⁻+2H⁺+2e ⁻  Reaction K

When such an oxidation reaction occurs, an oxidation reaction occurs inthe nitrous acid ions on the electrode 5 with respect to the reductionreaction of the nitric acid ions on the electrode 6 to cause a reductionin nitrogen treatment efficiency. According to the embodiment, however,the reaction K is suppressed because the surface area of at least theportion of the electrode 6 constituting the cathode immersed in thefor-treatment wastewater is larger than that of at least the portion ofthe electrode 5 constituting the anode immersed in the for-treatmentwastewater. Thus, a nitrogen treatment ability to reduce the nitric acidions to ammonia (ammonium ions) is improved.

On the other hand, on the platinum electrode 5 side constituting theanode, chloride ions as halide ions contained in the for-treatmentwastewater emit electrons to generate chlorine. The chlorine isdissolved in the water to generate hypochlorous acid simultaneously,ozone or active oxygen is generated.

In the case of the low concentrations of chloride ions contained in thefor-treatment wastewater, for example, halide ions such as chlorideions, iodide ions or bromide ions, or compounds containing such halideions, e.g., a sodium chloride or a potassium chloride, may be added tothe for-treatment wastewater. That is, the chloride ions of the sodiumchloride in the for-treatment wastewater are set to 10 ppm or higher to40000 ppm or lower.

The chloride ions originally contained in the for-treatment wastewateror the sodium chloride added as described above is oxidized on theelectrode 5 constituting the anode to generate chlorine (reaction L,shown in the case of the potassium chloride). The generated chlorinereacts with water in the for-treatment wastewater to generatehypochlorous acid (reaction M). Then, the generated hypochlorous acidreacts with the ammonia (ammonium ions) generated in the for-treatmentwastewater in the reaction J to make a plurality of chemical changes,and then converted into nitrogen gas (reaction N).

The reactions L to N are shown below.KCl→K⁺Cl⁻2Cl⁻→Cl₂+2e ⁻  Reaction LCl₂+H₂O→HClO+HCl  Reaction MNH₃+HClO→NH₂Cl+H₂O NH₂Cl+HClO→NHCl₂+H₂O NH₂Cl+NHCl₂→N₂⇑+3HCl  Reaction N

The ammonia (ammonium ions) in the for-treatment wastewater reacts withthe ozone or active oxygen generated on the electrode 5 sideconstituting the anode as shown in a reaction O to be denitrified tonitrogen gas.2NH₃(aq)+3(O)→N₂⇑+3H₂O  Reaction O

Accordingly, the nitrogen compounds of nitrate nitrogen, nitritenitrogen and ammonia nitrogen in the for-treatment wastewater can betreated in the same treating vessel 2.

The result of an experiment of FIG. 7 shows the concentration of nitratenitrogen in the for-treatment wastewater when a plate iron electrode ofa surface area 35 cm² is used for the electrode 6 constituting thecathode while a platinum wire of a surface area 0.5 cm² is used for theelectrode 5 constituting the anode. In the experiment, constant-currentelectrolysis was carried out in a mixed solution as the for-treatmentwastewater of KCl of 100 mM and KNO₃ of 10 mM by supplying 0.5 A betweenthe electrodes 5, 6 for five hours.

In FIG. 7, a white circle indicates the result of measuring theconcentration of nitrate nitrogen (NO₃ ⁻) for every one hour when aplatinum wire is used as an anode, and a black circle indicates the samewhen a platinum-iridium-based plate electrode is used as an anode. It isapparent from the drawing that treatment characteristics are exhibitedeven if any one of the plate platinum/iridium-based electrode and theplatinum wire electrode is used as the anode. However, there is a largedifference in treatment characteristics between the case of using theplatinum wire electrode as the anode and the case of using theplatinum/iridium-based plate electrode as the anode. That is, in thecase of using the platinum wire electrode as the anode, theconcentration of the nitrate nitrogen becomes equal to/lower than 100ppm of a wastewater quality standard after the passage of one hour.After the passage of five hours, the concentration of the nitratenitrogen becomes equal to/lower than 20 ppm. On the other hand, in thecase of using the plate platinum/iridium-based electrode as the anode,the concentration of the nitrate nitrogen becomes equal to/lower than 80ppm after the passage of three hours. After the passage of five hours,however, the treatment progresses only to the concentration of thenitrate nitrogen which is slightly lower than 70 ppm.

Accordingly, it can be understood that high treatment characteristics ofthe nitrate nitrogen in the for-treatment wastewater can be exhibited bysetting the surface area of the portion of the electrode 5 constitutingthe anode immersed in the for-treatment wastewater to be smaller thanthat of the portion of the electrode 6 constituting the cathode immersedin the for-treatment wastewater.

Thus, by setting the surface area of the portion of the electrode 6constituting the cathode immersed in the for-treatment wastewater to belarger than that of the portion of the electrode 5 constituting theanode immersed in the for-treatment wastewater, the reduction reactionof the nitrate nitrogen in the for-treatment wastewater to the nitridenitrogen and the ammonia is promoted on the electrode 6 constituting thecathode, whereby the time of the reduction reaction can be shortened.Additionally, low-concentration nitric acid ions can be treated.

After the end of the nitrogen compound treatment (nitrogen treatmentstep), the controller switches the polarities of potentials applied tothe electrodes 5, 6 (right side of FIG. 6, phosphorus treatment step,the nitrogen treatment reaction continues in the for-treatmentwastewater even after the switching of the polarities). Thus, theelectrode 5 constitutes a cathode while the electrode 6 constitutes ananode. Therefore, since the for-treatment wastewater is subjected toelectrolysis as an electrochemical technique, and the electrode 6constituting the anode is made of the conductor containing the iron asdescribed above, iron (II) ions are eluted from the electrode 6 into thefor-treatment wastewater to be oxidized to iron (III) ions therein.

The generated iron (III) ions are flocculated to settle with thephosphoric acid ions in the for-treatment wastewater by adephosphorization reaction as shown in a reaction P, whereby ironphosphate which has exsolution in water is generated.Fe³⁺+PO₄ ³⁻→FePO₄↓  Reaction P

Thus, the phosphoric acid ions as phosphorus compounds contained in thefor-treatment wastewater can be flocculated to settle as iron phosphate.

A portion of the ion (III) ions eluted in the state of the iron (II)ions in the for-treatment wastewater to supply electrons, and oxidizedon the electrode or in the for-treatment wastewater receives electronsagain on the electrode 5 side constituting the cathode in this case tobe reduced to iron (II) ions, and oxidized again on the electrode 6 sideconstituting the anode or in the for-treatment wastewater.

The technology for electrolyzing the for-treatment wastewater containingthe iron (II) ions to reduce the nitric acid ions contained in thefor-treatment wastewater to the ammonium ions is disclosed in the“Development of Inorganic Nitrogen Compound Treating Technology UsingElectrochemical Reaction” of the annual meeting lecture papers of JapanSociety on Water Environment.

Scales (CaCO₃, Mg(OH)₂ etc.) grown on the surface of the electrode 6constituting the cathode in the nitrogen treatment step are cleaned offfrom the surface when the electrode 6 becomes an anode in the phosphorustreatment step. Accordingly, the electrolytic performance of theelectrode 6 can be maintained high.

(4) Fourth Embodiment

Next, another embodiment of the present invention will be described byreferring to FIG. 8. FIG. 8 is an explanatory view showing an outline ofa wastewater treating apparatus 1 for realizing the wastewater treatingmethod of the fourth embodiment of the present invention. In thedrawings, reference numerals similar to those of FIG. 6 denote identicalor similar operations.

The wastewater treating apparatus 1 of the embodiment comprises atreating vessel 2 which constitutes a treating chamber 4 having an inletand an outlet (not shown) for the wastewater therein, a pair ofelectrodes 5 (first electrode), 6 (second electrode) oppositely arrangedto be partially immersed in the for-treatment wastewater of the treatingchamber 4, an electrode 7 (third electrode) arranged to be immersed atleast partially in the for-treatment wastewater, an unshown power supplyto energize the electrodes 5, 6, 7, a controller to control the powersupply, an ammonia concentration detection sensor 8 for detectingammonia and ammonium ion concentrations in the for-treatment wastewater,etc. Agitation means may be disposed in the treating vessel 2 to agitatethe contents therein.

The electrode 6 is made of, for example, iron (Fe) or a conductorcovered with the iron. Each of the electrodes 5 and 7 is, for example, anoble metal electrode made of platinum (Pt) or a mixture of platinum andiridium (Ir), or made of an insoluble conductor covered with such anelement. In addition, as a conductor capable of generatinghypohalogeneous acid such as hypochlorous acid, ozone or active oxygen,for example, a ceramic-based conductor, an insoluble conductor coveredwith the ceramic-based conductor, a stainless steel, an insolubleconductor covered with the stainless, a carbon-based conductor, or aninsoluble conductor covered with the carbon-based conductor may be used.In this case, the conductor can be prepared at low cost. Especially,durability can be improved when the ceramic-based conductor or theconductor such as a stainless steel is used.

According to the embodiment, the electrode 6 is made of a plate memberwhile the electrode 5 is made of a linear member such as a wire so thata surface area of at least a portion of the electrode 6 immersed in thefor-treatment wastewater can be larger than that of at least a portionof the electrode 5 immersed in the for-treatment wastewater. Accordingto the embodiment, the electrode 7 is also made of a plate member sothat at least a portion of the electrode 7 immersed in the treatmentwater can be larger than that of at least the portion of the electrodeimmersed in the for-treatment wastewater. According to the embodiment,platinum is used for both of the electrodes 5 and 7.

Thus, the for-treatment wastewater which contains nitric acid ions asnitrate nitrogen and phosphoric acid ions as phosphorus compounds isreserved in the treating chamber 4 of the treating vessel 2. The powersupply is turned ON by the controller to apply a positive potential tothe electrode 5 and a negative potential to the electrode 6 (firsttreatment step, left side of FIG. 8). At this time, no potential isapplied to the electrode 7. Accordingly, the electrode 5 becomes ananode while the electrode 6 becomes a cathode.

As a result of the application of the potentials, electrons generated onthe electrode 5 side constituting the anode are fed to the electrode 6side constituting the cathode, and the nitric acid ions contained as thenitrate nitrogen in the for-treatment wastewater are reduced to nitrousacid ions (the reaction I). Further, electrons are fed to the electrode6 side constituting the cathode to reduce the nitrate nitrogen reducedto the nitrous acid ions to ammonia (ammonium ions) (the reaction J).

In this case, when nitrous acid ions generated by reducing the nitricacid ions on the electrode 6 constituting the cathode transfer to theelectrode 5 side constituting the anode, an oxidation reaction occurs onthe electrode 5 constituting the anode to oxidize the nitrous acid ionsto nitric acid ions (the reaction K).

When such an oxidation reaction occurs, an oxidation reaction occurs inthe nitrous acid ions on the electrode 5 with respect to the reductionreaction of the nitric acid ions on the electrode 6 to cause a reductionin nitrogen treatment efficiency. According to the embodiment, however,the reaction K is suppressed because the surface area of at least theportion of the electrode 6 constituting the cathode immersed in thefor-treatment wastewater is larger than that of at least the portion ofthe electrode 5 constituting the anode immersed in the for-treatmentwastewater. Thus, a nitrogen treatment ability to reduce the nitric acidions to ammonia (ammonium ions) is improved.

The ammonia concentration detector sensor 8 detects ammonia or ammoniumion concentrations in the for-treatment wastewater. When the ammonia andammonium ion concentrations in the for-treatment wastewater reachpredetermined values, the first treatment step is finished.

In addition to the ammonia concentration detection sensor 8, a nitricacid ion concentration detection sensor (not shown) may be disposed inthe wastewater treating apparatus 1 to detect concentrations of nitricacid ions in the for-treatment wastewater. In a state in which thenitric acid ion concentration detection sensor detects that the nitricacid ions have almost been removed from the for-treatment wastewater,the ammonia and ammonium ion concentrations in the for-treatmentwastewater may be detected by the ammonia concentration detection sensor8, and the first treatment step may be finished when the ammonia andammonium ion concentrations in the for-treatment wastewater are equalto/higher than the predetermined values.

After the end of the first treatment step, the controller applies anegative potential to the electrode 6 and a positive electrode to theelectrode 7 (second treatment step, center of FIG. 8). At this time, nopotential is applied to the electrode 5. Accordingly, the electrode 7becomes an anode while the electrode 6 becomes a cathode.

In this case, on the platinum electrode 7 constituting the anode,chloride ions contained in the for-treatment wastewater emit electronsto generate chlorine. The chlorine is dissolved in the water to generatehypochlorous acid (the reactions L to M). Simultaneously, ozone oractive oxygen is generated.

The generated hypochlorous acid reacts with the ammonia (ammonium ions)generated in the for-treatment wastewater in the reaction J of the firsttreatment step to make a plurality of chemical changes, and thenconverted into nitrogen gas (the reaction N). In the first treatmentstep, the ammonia (ammonium ions) in the for-treatment wastewater reactswith the ozone or active oxygen generated on the electrode 5 sideconstituting the anode as shown in the reaction O to be denitrified tonitrogen gas.

According to the embodiment, in the second treatment step, the negativepotential is applied to the electrode 6 while the positive potential isapplied to the electrode 7. However, similar effects can be obtained byapplying a negative potential to the electrode 6 and positive potentialsto the electrodes 7 and 5.

Accordingly, the nitrogen compounds of nitrate nitrogen, nitritenitrogen and ammonia nitrogen in the for-treatment wastewater can betreated in the same treating vessel 2.

Thus, since the first treatment step is switched to the second treatmentstep by the detection of the ammonia concentration detector sensor 8, anoxidation reaction from nitrous acid ions to nitric acid ions can besuppressed. Moreover, while the ammonia and ammonium ions cannot becompletely removed on the electrode 5 as the first electrode, byswitching to the electrode 7 as the third electrode, the ammonia and theammonium ions in the for-treatment wastewater can be quickly convertedinto nitrogen gas.

After the end of such treatment of the nitrogen compounds (first andsecond treatment steps), the controller applies a negative potential tothe electrode 7 and a positive potential to the electrode 6. At thistime, no potential is applied to the electrode 5 (right side of FIG. 8,third treatment step, the nitrogen treatment continues in thefor-treatment wastewater even after the switching of potentialapplication). Thus, the electrode 7 constitutes a cathode while theelectrode 6 constitutes an anode. Therefore, since the for-treatmentwastewater is subjected to electrolysis as an electrochemical technique,and the electrode 6 constituting the anode is made of the conductorcontaining the iron as described above, iron (II) ions are eluted fromthe electrode 6 into the for-treatment wastewater to be oxidized to iron(III) ions therein.

The generated iron (III) ions are flocculated to settle with thephosphoric acid ions in the for-treatment wastewater by adephosphorization reaction as shown in the reaction P, whereby ironphosphate which has exsolution in water is generated. Thus, thephosphoric acid ions as phosphorus compounds contained in thefor-treatment wastewater can be flocculated to settle as iron phosphate.

According to the embodiment, in the third treatment step, the negativepotential is applied to the electrode 7 while the positive potential isapplied to the electrode 6. In addition, similar effects can obtainedwhen negative potentials are applied to the electrodes 7 and 5, and apositive potential is applied to the electrode 6.

If only the nitrogen treatment step is carried out in the thirdembodiment, or only the first and second treatment steps are carried outin the fourth embodiment, the electrode 6 may be made of a stainlesssteel. In this case, as the stainless steel, there are available anaustenite-based steel type (SUS304), an austenite/ferrite-based steeltype (SUS329J1), a ferrite-based steel type (SUS444), a martensite-basedsteel type (SUS410), a precipitation hardened steel type (SUS631) etc.The aforementioned effects of the nitrogen compound treatment can beexpected when such a stainless steel is used. In this case, especially,durability can be improved by preventing the generation of rust on theelectrodes.

According to each of the foregoing embodiments, the nitrogen compoundsand the phosphorus compounds in the for-treatment wastewater can beeffectively treated by the electrochemical technique. Thus, the nitrogencompounds and the phosphorus compounds in the for-treatment wastewatercan be treated in the same treating vessel to eliminate the conventionalnecessity of installing a large biological treating vessel, and it ispossible to prevent increases in facility construction cost andapparatus installation area.

Furthermore, it is possible to eliminate the necessity of complexmaintenance work of denitrification bacteria which is necessary in thebiological treatment, and to provide stable and high nitrogen andphosphorus treating efficiency.

According to each of the embodiments, the halide ions contained in thefor-treatment wastewater are chloride ions to generate hypochlorousacid. In addition, similar effects can be obtained even when otherhypohalogeneous acid is generated by fluoride ions, bromide ions or thelike.

As described above, according to the present invention, when thenitrogen compounds in the for-treatment wastewater are treated, at leastportions of the pair of electrodes are immersed in the for-treatmentwastewater, the material of one of the electrodes constituting the anodeis an insoluble conductor, the material of the other electrodeconstituting the cathode is an element in the group VIII of the periodictable, a conductor containing the element in the group VIII, or aconductor covered with the element in the same group or the conductorcontaining the element in the same group, and the for-treatmentwastewater is treated by the electrochemical technique. Thus, on theelectrode constituting the cathode, the reduction reaction of thenitrate nitrogen in the for-treatment wastewater to the nitrous acidnitrogen or ammonia is promoted. As a result, the time of reductionreaction can be shortened, and low-concentration nitric acid ions can betreated.

In this case, since the material of the one electrode constituting theanode is the insoluble conductor, elution from the electrode itself canbe prevented to improve the durability of the electrode. Moreover, sincethere is no great amount of sludge generated by the great amount of ironeluted into the for-treatment wastewater different from the conventionalcase, the workability of waste disposal maintenance and adaptability tothe environment can be improved.

Especially, in this case, since the other electrode constituting thecathode is made of the material which is the element in the group VIIIof the periodic table, the conductor containing the element in the groupVIII, the conductor covered with the element in the same group or theconductor containing the element in the same group, and an alloycontaining copper is not used, the problem of toxicity caused by theelution of copper in the for-treatment wastewater can be solved.

According to the present invention, by removing the ammonia nitrogengenerated in the for-treatment wastewater, the nitrogen compounds in thefor-treatment wastewater can be efficiently treated.

According to the present invention, the for-treatment wastewater ispartitioned into the region of the one electrode constituting the anodeand the region of the other electrode constituting the cathode by thecation-exchange membrane. Thus, the nitrous acid ions generated on theother electrode constituting the cathode transfer to the one electrodeside constituting the anode to prevent the inconvenience of oxidationthereof. As a result, the efficiency of the reduction reaction of thenitrate nitrogen in the for-treatment wastewater to the nitrous acidnitrogen and ammonia can be further improved.

According to the present invention, the ammonia removal treatment stepof removing ammonia nitrogen generated in the for-treatment wastewaterfrom the for-treatment wastewater by the aforementioned treatment, andthe phosphorus removal treatment step of switching the polarities of theelectrodes to treat the phosphorus compounds in the for-treatmentwastewater by an electrochemical technique are carried out. Thus, theammonia nitrogen generated in the aforementioned treatment can beefficiently treated in the ammonia removal treatment step.

By carrying out the phosphorus removal step of switching the polaritiesof the electrodes to treat the phosphorus compounds in the for-treatmentwastewater by the electrochemical technique, for example, iron (II) ionsin the group VIII of the periodic table are eluted in the for-treatmentwastewater by the other electrode constituting the anode, the iron (III)ions generated by oxidation in the for-treatment wastewater arechemically reacted with the phosphoric acid ions as the phosphoruscompounds in the for-treatment wastewater, and flocculated to settle asiron phosphate.

According to the present invention, in addition, the material of the oneelectrode constituting the anode is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique. Thus, on the other electrode constituting the cathode, thereduction reaction of the nitrate nitrogen in the for-treatmentwastewater to the nitrous acid nitrogen or ammonia is promoted. As aresult, the time of the reduction reaction can be shortened, andlow-concentration nitric acid ions can be treated.

Since the ammonia generated on the other electrode constituting thecathode is subjected to denitrification reaction with the substance suchas hypochlorous acid as hypohalogeneous acid generated on the oneelectrode constituting the anode, it is possible to effectively removethe nitrate nitrogen, the ammonia nitrogen and the nitrogen componentssuch as nitrogen compounds by a synergy effect. Thus, the nitrogencompounds can be efficiently removed from the for-treatment wastewaterwhich contains the nitrogen compounds discharged from generalhouseholds, factories or the like to improve the nitrogen compoundtreating ability.

According to the present invention, after the completion of theaforementioned treatment, the polarities of the electrodes are switched,and the phosphorus compounds in the for-treatment wastewater are treatedby the electrochemical technique. Thus, for example, iron (II) ions asthe element in the group VIII of the periodic table are eluted into thefor-treatment wastewater from the other electrode constituting theanode, the iron (III) ions generated by oxidation in the for-treatmentwastewater are chemically reacted with the phosphoric acid ions as thephosphorus compounds in the for-treatment wastewater, and flocculated tosettle as iron phosphate.

Thus, the phosphorus compounds in the for-treatment wastewater can betreated.

According to the present invention, when the nitrogen compounds in thefor-treatment wastewater are treated, the for-treatment wastewater ispartitioned into one region and the other region by the cation-exchangemembrane, and the first treatment step is carried out in which at leasta portion of the first electrode is immersed in the one region; at leasta portion of the second electrode is immersed in the other region; thefirst electrode constituting an anode is a conductor; a material of thesecond electrode constituting a cathode is an element in the group VIIIof the periodic table, a conductor containing the element in the groupVIII, or a conductor covered with the element in the same group or theconductor containing the element in the same group; and thefor-treatment wastewater is treated by an electrochemical technique.Thus, on the second electrode constituting the cathode, the reductionreaction of the nitrate nitrogen in the for-treatment wastewater to thenitrite nitrogen or ammonia is promoted. As a result, the time ofreduction reaction can be shortened, and low-concentration nitric acidions can be treated.

In this case, the for-treatment wastewater is partitioned into theregion in which the first electrode constituting the anode is immersedand the region in which the second electrode constituting the cathode isimmersed by the cation-exchange membrane. Thus, the nitrous acid ionsgenerated on the second electrode constituting the cathode transfer tothe first electrode side constituting the anode to prevent theinconvenience of oxidation thereof. As a result, the efficiency of thereduction reaction of the nitrate nitrogen in the for-treatmentwastewater to the nitrous acid nitrogen and ammonia can be furtherimproved.

Additionally, the second treatment step is carried out in which at leasta portion of the third electrode is immersed in the other region; amaterial of the third electrode is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by an electrochemicaltechnique; the third electrode is used as the anode; the first electrodeis used as the cathode; and the for-treatment wastewater treated by thefirst treatment step is treated by the electrochemical technique.Accordingly, since the ammonia generated on the second electrodeconstituting the cathode in the first treatment step is subjected todenitrification reaction with the substance such as hypochlorous acid ashypohalogeneous acid generated on the third electrode constituting theanode in the second treatment step, it is possible to effectively removethe nitrate nitrogen, the ammonia nitrogen and the nitrogen componentssuch as nitrogen compounds by a synergy effect. Thus, the nitrogencompounds can be efficiently removed from the for-treatment wastewaterwhich contains the nitrogen compounds discharged from generalhouseholds, factories or the like to improve the nitrogen compoundtreating ability.

Especially, in this case, since the second electrode constituting thecathode is made of the material which is the element in the group VIIIof the periodic table, the conductor containing the element in the groupVIII, the conductor covered with the element in the same group or theconductor containing the element in the same group, and an alloycontaining copper is not used, the problem of toxicity caused by theelution of copper in the for-treatment wastewater can be solved.

According to the present invention, after the completion of the secondtreatment, the third treatment step is carried out in which thephosphorus compounds in the for-treatment wastewater are treated by theelectrochemical technique while the first electrode is set as thecathode. Thus, for example, iron (II) ions as the element in the groupVIII of the periodic table are eluted into the for-treatment wastewaterfrom the second electrode constituting the anode in the third treatmentstep, the iron (III) ions generated by oxidation in the for-treatmentwastewater are chemically reacted with the phosphoric acid ions as thephosphorus compounds in the for-treatment wastewater, and flocculated tosettle as iron phosphate.

Thus, the phosphorus compounds in the for-treatment wastewater can betreated.

According to the present invention, the conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique is a noble metal, a conductor covered with the noble metal, aceramic conductor, a ferrite, a carbon-based conductor or a stainlesssteel. Thus, the hypohalogeneous acid, the ozone or the active oxygencan be easily generated in the for-treatment wastewater, and thedenitrification treatment of the ammonia or the ammonium ions can beeffectively carried out.

According to the present invention, when the nitrogen compounds in thefor-treatment wastewater are treated, at least portions of the pair ofelectrodes are immersed in the for-treatment wastewater, the material ofone of the electrodes constituting the anode is a conductor capable ofgenerating hypohalogeneous acid, ozone or active oxygen by theelectrochemical technology, the material of the other electrodeconstituting the cathode is a conductor containing iron, and thefor-treatment wastewater is treated by the electrochemical technique.Thus, on the other electrode constituting the cathode, the reductionreaction of the nitrate nitrogen in the for-treatment wastewater to thenitrite nitrogen or ammonia is promoted. As a result, the time ofreduction reaction can be shortened, and low-concentration nitric acidions can be treated.

The surface area of at least the portion of the other electrodeconstituting the cathode immersed in the for-treatment wastewater is setto be larger than that of the portion of the one electrode constitutingthe anode immersed in the for-treatment wastewater, and thefor-treatment wastewater is treated by the electrochemical technique.Thus, the oxidation reaction of the nitrogen compounds generated on theone electrode constituting the anode, especially from the nitritenitrogen to the nitrate nitrogen, reduced, and accordingly the reductionreaction from the nitrate nitrogen to the nitrite nitrogen on the otherelectrode constituting the cathode is promoted. Thus, the reductionreaction from the nitrate nitrogen to the nitrite nitrogen or theammonia is promoted much more to enable the shortening of the time ofthe reduction reaction, and the treatment of low-concentration nitricacid ions.

Since the ammonia generated on the other electrode constituting thecathode is subjected to denitrification reaction with the substance suchas hypochlorous acid as hypohalogeneous acid generated on the oneelectrode constituting the anode, it is possible to effectively removethe nitrate-nitrogen, the ammonia nitrogen and the nitrogen componentssuch as nitrogen compounds by a synergy effect. Thus, the nitrogencompounds can be efficiently removed from the for-treatment wastewaterwhich contains the nitrogen compounds discharged from generalhouseholds, factories or the like to improve the nitrogen compoundtreating ability.

According to the present invention, after the completion of theaforementioned treatment, the polarities of the electrodes are switched,and the phosphorus compounds in the for-treatment wastewater are treatedby the electrochemical technique. Thus, iron (II) ions are eluted intothe for-treatment wastewater from the other electrode constituting theanode, the iron (III) ions generated by oxidation in the for-treatmentwastewater are chemically reacted with the phosphoric acid ions as thephosphorus compounds in the for-treatment wastewater, and flocculated tosettle as iron phosphate.

Thus, the phosphorus compounds in the for-treatment wastewater can betreated.

According to the present invention, the wastewater treating method fortreating nitrogen compounds in the for-treatment wastewater, comprises:the first treatment step in which at least portions of the first andsecond electrodes are immersed in the for-treatment wastewater; thematerial of the first electrode constituting an anode is a conductor;the material of the second electrode constituting a cathode is aconductor containing iron; the surface area of the portion of the secondelectrode which is at least immersed in the for-treatment wastewater isset to be larger than that of the portion of the first electrode whichis at least immersed in the for-treatment wastewater; and thefor-treatment wastewater is treated by the electrochemical technique;and the second treatment step in which at least a portion of the thirdelectrode is immersed in the for-treatment wastewater; the material ofthe third electrode is a conductor capable of generating hypohalogenousacid, ozone or active oxygen by the electrochemical technique, when thethird electrode is used as an anode; the surface area of the portion ofthe third electrode which is at least immersed in the for-treatmentwastewater is set to be larger than that of the portion of the firstelectrode which is at least immersed in the for-treatment wastewater;and after completion of the first treatment step, the third electrode isused as the anode; the second electrode is used as the cathode; and thefor-treatment wastewater are treated by the electrochemical technique.Thus, on the second electrode constituting the cathode, the reductionreaction of the nitrate nitrogen in the for-treatment wastewater to thenitrite nitrogen or ammonia is promoted. As a result, the time ofreduction reaction can be shortened, and low-concentration nitric acidions can be treated.

The surface area of at least the portion of the second electrodeconstituting the cathode immersed in the for-treatment wastewater is setto be larger than that of the portion of the first electrodeconstituting the anode immersed in the for-treatment wastewater, and thefor-treatment wastewater is treated by the electrochemical technique.Thus, the oxidation reaction of the nitrogen compounds generated on thefirst electrode constituting the anode, especially from the nitritenitrogen to the nitrate nitrogen, is reduced, and accordingly thereduction reaction from the nitrate nitrogen to the nitrite nitrogen onthe second electrode constituting the cathode can be promoted. Thus, thereduction reaction from the nitrate nitrogen to the nitrite nitrogen orthe ammonia is promoted much more to enable the shortening of the timeof the reduction reaction, and the treatment of low-concentration nitricacid ions.

Additionally, the second treatment step is carried out in which at leasta portion of the third electrode is immersed in the other region; thematerial of the third electrode is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique; the surface of at least the portion of the third electrodeimmersed in the for-treatment wastewater is set to be larger than thatof at least the portion of the first electrode immersed in thefor-treatment wastewater; the third electrode is used as the anode; thesecond electrode is used as the cathode; and the for-treatmentwastewater after the completion of the first treatment step is treatedby the electrochemical technique. Accordingly, since the ammoniagenerated on the second electrode constituting the cathode in the firsttreatment step is subjected to denitrification reaction with thesubstance such as hypochlorous acid as hypohalogeneous acid generated onthe third electrode constituting the anode in the second treatment step,it is possible to effectively remove the nitrate nitrogen, the ammonianitrogen and the nitrogen components such as nitrogen compounds by asynergy effect. Thus, the nitrogen compounds can be efficiently removedfrom the for-treatment wastewater which contains the nitrogen compoundsdischarged from general households, factories or the like to improve thenitrogen compound treating ability.

Especially, in this case, since the surface area of at least the portionof the third electrode constituting the anode immersed in thefor-treatment wastewater is set to be larger than that of at least theportion of the thirst electrode immersed in the for-treatment wastewaterin the second treatment step, the generation of a substance such ashypohalogeneous acid generated on the third electrode constituting theanode in the second treatment step can be promoted. Thus, thedenitrification reaction with the ammonia nitrogen in the for-treatmentwastewater can be promoted. As a result, it is possible to moreeffectively remove the nitrate nitrogen, the ammonia nitrogen and thenitrogen components such as nitrogen compounds.

According to the present invention, the operation transfers from thefirst treatment step to the second treatment step, when ammonia andammonium ions in the for-treatment wastewater reach the predeterminedconcentrations. Thus, after the nitrate nitrogen and the nitrous acidnitrogen in the for-treatment wastewater are subjected to reductionreaction on the second electrode constituting the cathode to reduce thenitrate nitrogen in the for-treatment wastewater to the ammonia andammonium ions in the first treatment step, the operation can transfer tothe second treatment step.

Thus, after the nitrate nitrogen and the hypohalogeneous nitrogen in thefor-treatment wastewater have been sufficiently reduced to the ammoniaand ammonium ions, in the second treatment step, ammonia denitrificationreaction can be carried out by a substance such as hypohalogeneous ionsgenerated on the third electrode constituting the anode. As a result, itis possible to more effectively remove the nitrate nitrogen, the ammonianitrogen and the nitrogen components such as nitrogen compounds.

According to the present invention, the third treatment step is carriedout in which after the completion of the second treatment step, thethird electrode is used as the cathode; the second electrode is used asthe anode; and the phosphorus compounds in the for-treatment wastewaterare treated by the electrochemical technique. Thus, in the thirdtreatment step, iron (II) ions can be eluted into the for-treatmentwastewater from the second electrode constituting the anode, the iron(III) ions generated by oxidation in the for-treatment wastewater can bechemically reacted with the phosphoric acid ions as the phosphoruscompounds in the for-treatment wastewater, and flocculated to settle asiron phosphate.

Thus, the phosphorus compounds in the for-treatment wastewater can betreated.

According to the present invention, in each of the foregoing inventions,a stainless steel is used as the material of the other electrode or thesecond electrode. Thus, it is possible to improve the durability of theother electrode or the second electrode in the treatment of the nitrogencompounds in the for-treatment water.

According to the present invention, the conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique is a noble metal or a conductor covered with the noble metal.Thus, hypohalogeneous acid, ozone or active oxygen can be easilygenerated in the for-treatment wastewater, and the denitificationreaction of ammonia or ammonium ions can be effectively carried out.

According to the present invention, the conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique is a ceramic-based conductor or a conductor covered with theceramic-based conductor. Thus, hypohalogeneous acid, ozone or activeoxygen can be easily generated in the for-treatment wastewater, and thedenitrification reaction of ammonia or ammonium ions can be effectivelycarried out.

Additionally, the use of the ceramic-based conductor enables preparationof a durable conductor at low cost.

According to the present invention, the conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique is a carbon-based conductor or a conductor covered with thecarbon-based conductor. Thus, hypohalogeneous acid, ozone or activeoxygen can be easily generated in the for-treatment wastewater, and thedenitrification reaction of ammonia or ammonium ions can be effectivelycarried out.

Additionally, the use of the carbon-based conductor enables preparationof a conductor at low cost.

According to the present invention, the conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique is a stainless steel or a conductor covered with the stainlesssteel. Thus, hypohalogeneous acid, ozone or active oxygen can be easilygenerated in the for-treatment wastewater, and the denitrificationreaction of ammonia or ammonium ions can be effectively carried out.

Additionally, the use of the stainless steel enables preparation of adurable conductor at low cost.

1. A wastewater treating method for treating nitrogen compounds in afor-treatment wastewater by an electrochemical technique comprising: anammonia removal treatment step of removing ammonia nitrogen produced inthe for-treatment wastewater from the for-treatment wastewater, and aphosphorus removal treatment step of switching polarities of theelectrodes to treat phosphorus compounds in the for-treatment wastewaterby an electrochemical technique, wherein at least portions of a pair ofelectrodes are immersed in the for-treatment wastewater, a material ofone of the electrodes constituting an anode is an insoluble conductor,and a material of the other electrode constituting a cathode is anelement in the group VIII of the periodic table, a conductor containingthe element in the group VIII, or a conductor covered with the elementin the same group or the conductor containing the element in the samegroup.
 2. The wastewater treating method according to claim 1, whereinthe for-treatment wastewater is partitioned into a region of the oneelectrode constituting the anode and a region of the other electrodeconstituting the cathode by a cation-exchange membrane.
 3. Thewastewater treating method according to claim 1, wherein hypohalogenousacid is added to the for-treatment wastewater in the ammonia removaltreatment step.
 4. The wastewater treating method according to claim 1,wherein the material of the one electrode constituting the anode is aconductor capable of generating hypohalogenous acid, ozone or activeoxygen by an electrochemical technique.
 5. The wastewater treatingmethod according to claim 4, wherein after completion of the treatmentaccording to claim 1, polarities of the electrodes are switched, andphosphorus compounds in the for-treatment wastewater are treated by anelectrochemical technique.
 6. A wastewater treating method for treatingnitrogen compounds in a for-treatment wastewater, wherein thefor-treatment wastewater is partitioned into one region and the otherregion by a cation-exchange membrane, the method comprising: a firsttreatment step in which at least a portion of a first electrode isimmersed in the one region; at least a portion of a second electrode isimmersed in the other region; the first electrode constituting an anodeis a conductor; a material of the second electrode constituting acathode is an element in the group VIII of the periodic table, aconductor containing the element in the group VIII, or a conductorcovered with the element in the same group or the conductor containingthe element in the same group; and the for-treatment wastewater istreated by an electrochemical technique, and a second treatment step inwhich at least a portion of a third electrode is immersed in the otherregion; a material of the third electrode is a conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by anelectrochemical technique; the third electrode is used as the anode; thefirst electrode is used as the cathode; and the for-treatment wastewatertreated by the first treatment step is treated by the electrochemicaltechnique.
 7. The wastewater treating method according to claim 6,further comprising a third treatment step in which after completion ofthe second treatment step, the second electrode is used as the anode;the first electrode is used as the cathode; and phosphorus compounds inthe for-treatment wastewater are treated by the electrochemicaltechnique.
 8. The wastewater treating method according to claim 4, 5, 6or 7, wherein the conductor capable of generating hypohalogenous acid,ozone or active oxygen by the electrochemical technique is a noblemetal, a conductor covered with the noble metal, a ceramic conductor, acarbon-based conductor or a stainless steel.
 9. The wastewater treatingmethod according to claim 8, wherein the ceramic conductor is a ferrite.10. A wastewater treating apparatus for treating nitrogen compounds in afor-treatment wastewater, comprising a pair of electrodes which is atleast partially immersed in the for-treatment wastewater and means forswitching polarities of the electrodes to treat phosphorus compounds inthe for-treatment wastewater by an electrochemical technique, wherein amaterial of one of the electrodes is an insoluble conductor; a materialof the other electrode is an element in the group VIII of the periodictable, a conductor containing the element in the group VIII, or aconductor covered with the element in the same group or the conductorcontaining the element in the same group; and the one electrode is usedas an anode; the other electrode is used as a cathode; and thefor-treatment wastewater is treated by an electrochemical technique. 11.The wastewater treating apparatus according to claim 10, wherein thematerial of the one electrode is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by an electrochemicaltechnique.
 12. A wastewater treating apparatus for treating nitrogencompounds in a for-treatment wastewater, comprising: a cation-exchangemembrane for partitioning the for-treatment wastewater into one regionand the other region, a first electrode which is at least partiallyimmersed in the one region, and a second electrode and a third electrodewhich are at least partially immersed in the other region, wherein amaterial of the first electrode is a conductor; a material of the secondelectrode is an element in the group VIII of the periodic table, aconductor containing the element in the group VIII, or a conductorcovered with the element in the same group or the conductor containingthe element in the same group; and a material of the third electrode isa conductor capable of generating hypohalogenous acid, ozone or activeoxygen by an electrochemical technique; the apparatus being adapted toperform a first treatment step in which the first electrode is used asan anode; the second electrode is used as a cathode; and thefor-treatment wastewater are treated by the electrochemical technique,and a second treatment step in which after completion of the firsttreatment step, the third electrode is used as the anode; the firstelectrode is used as the cathode; and the for-treatment wastewater istreated by the electrochemical technique.
 13. The wastewater treatingapparatus according to claim 12, which is adapted to perform a thirdtreatment step in which after completion of the second treatment step,the second electrode is used as the anode; the first electrode is usedas the cathode; and phosphorus compounds in the for-treatment wastewaterare treated by the electrochemical technique.
 14. The wastewatertreating apparatus according to claim 11, 12 or 13, wherein theconductor capable of generating hypohalogenous acid, ozone or activeoxygen by the electrochemical technique is a noble metal, a conductorcovered with the noble metal, a ceramic conductor, a carbon-basedconductor or a stainless steel.
 15. The wastewater treating apparatusaccording to claim 14, wherein the ceramic conductor is a ferrite.
 16. Awastewater treating method for treating nitrogen compounds in afor-treatment wastewater, wherein at least portions of a pair ofelectrodes are immersed in the for-treatment wastewater, a material ofone electrode constituting an anode is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by an electrochemicaltechnique, a material of the other electrode constituting a cathode is aconductor containing iron, and a surface area of a portion of the otherelectrode which is at least immersed in the for-treatment wastewater isset to be larger than a surface area of a portion of the one electrodewhich is at least immersed in the for-treatment wastewater; and thefor-treatment wastewater is treated by the electrochemical technique.17. The wastewater treating method according to claim 16, wherein aftercompletion of the treatment according to claim 18, polarities of theelectrodes are switched; and phosphorus compounds in the for-treatmentwastewater are treated by an electrochemical technique.
 18. A wastewatertreating method for treating nitrogen compounds in a for-treatmentwastewater, comprising: a first treatment step in which at leastportions of first and second electrodes are immersed in thefor-treatment wastewater; a material of the first electrode constitutingan anode is a conductor; a material of the second electrode constitutinga cathode is a conductor containing iron; a surface area of a portion ofthe second electrode which is at least immersed in the for-treatmentwastewater is set to be larger than a surface area of a portion of thefirst electrode which is at least immersed in the for-treatmentwastewater; and the for-treatment wastewater is treated by theelectrochemical technique; a second treatment step in which at least aportion of a third electrode is immersed in the for-treatmentwastewater; a material of the third electrode is a conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by anelectrochemical technique, when the third electrode is used as an anode;a surface area of a portion of the third electrode which is at leastimmersed in the for-treatment wastewater is set to be larger than asurface area of a portion of the first electrode which is at leastimmersed in the for-treatment wastewater; and after completion of thefirst treatment step, the third electrode is used as the anode; thesecond electrode is used as the cathode; and the for-treatmentwastewater are treated by the electrochemical technique.
 19. Thewastewater treating method according to claim 18, wherein an operationtransfers from the first treatment step to the second treatment step,when ammonia and ammonium ions in the for-treatment wastewater havereached predetermined concentrations.
 20. The wastewater treating methodaccording to claim 18 or 19, further comprising a third treatment stepin which after completion of the second treatment step, the thirdelectrode is used as the cathode; the second electrode is used as theanode; and phosphorus compounds in the for-treatment wastewater aretreated by the electrochemical technique.
 21. The wastewater treatingmethod according to claim 16, 17, 18 or 19, wherein a stainless steel isused as the material of the other electrode or the second electrode. 22.The wastewater treating method according to claim 16, 17, 18 or 19,wherein as the conductor capable of generating hypohalogenous acid,ozone or active oxygen by the electrochemical technique, there is used anoble metal or a conductor covered with the noble metal.
 23. Thewastewater treating method according to claim 16, 17, 18 or 19, whereinas the conductor capable of generating hypohalogenous acid, ozone oractive oxygen by the electrochemical technique, there is used a ceramicconductor or a conductor covered with the ceramic conductor.
 24. Thewastewater treating method according to claim 16, 17, 18 or 19, whereinas the conductor capable of generating hypohalogenous acid, ozone oractive oxygen by the electrochemical technique, there is used acarbon-based conductor or a conductor covered with the carbon-basedconductor.
 25. The wastewater treating method according to claim 16, 17,18 or 19, wherein as the conductor capable of generating hypohalogenousacid, ozone or active oxygen by the electrochemical technique, there isused a stainless steel or a conductor covered with the stainless steel.26. A wastewater treating apparatus for treating nitrogen compounds in afor-treatment wastewater, comprising a pair of electrodes which is atleast partially immersed in the for-treatment wastewater, wherein amaterial of one of the electrodes is a conductor capable of generatinghypohalogenous acid, ozone or active oxygen by an electrochemicaltechnique; a material of the other electrode is a conductor containingiron; and a surface area of a portion of the other electrode which is atleast immersed in the for-treatment wastewater is set to be larger thana surface area of a portion of the one electrode which is at leastimmersed in the for-treatment wastewater, and the one electrode is usedas an anode; the other electrode is used as a cathode; and thefor-treatment wastewater is treated by the electrochemical technique.27. The wastewater treating apparatus according to claim 26, furthercomprising means for switching polarities of the electrodes to treatphosphorus compounds in the for-treatment wastewater by anelectrochemical technique.
 28. A wastewater treating apparatus fortreating nitrogen compounds in a for-treatment wastewater, comprisingfirst, second and third electrodes which are at least partially immersedin the for-treatment wastewater, wherein the first electrode is aconductor; a material of the second electrode is a conductor containingiron; a material of the third electrode is a conductor capable ofgenerating hypohalogenous acid, ozone or active oxygen by anelectrochemical technique when the third electrode is used as an anode;and a surface area of portions of the second and third electrodes whichare at least immersed in the for-treatment wastewater is set to belarger than a surface area of a portion of the first electrode which isat least immersed in the for-treatment wastewater, the apparatus beingadapted to perform a first treatment step in which the first electrodeis used as an anode; the second electrode is used as a cathode; and thefor-treatment wastewater is treated by the electrochemical technique;and a second treatment step in which after completion of the firsttreatment step, the third electrode is used as the anode; the secondelectrode is used as the cathode; and the for-treatment wastewater istreated by the electrochemical technique.
 29. The wastewater treatingapparatus according to claim 28, further comprising means for detectingammonia and ammonium ions in the for-treatment wastewater, wherein whenammonia and ammonium ions in the for-treatment wastewater have reachedpredetermined concentrations, an operation transfers from the firsttreatment step to the second treatment step.
 30. The wastewater treatingapparatus according to claim 28 or 29, which is adapted to perform athird treatment step in which after completion of the second treatmentstep, the third electrode is used as the cathode; the second electrodeis used as the anode; and phosphorus compounds in the for-treatmentwastewater are treated by the electrochemical technique.
 31. Thewastewater treating apparatus according to claim 26, 27, 28 or 29,wherein a material of the other electrode or the second electrode is astainless steel.
 32. The wastewater treating apparatus according toclaim 26, 27, 28 or 29, wherein the conductor capable of generatinghypohalogenous acid, ozone or active oxygen by the electrochemicaltechnique is a noble metal or a conductor covered with the noble metal.33. The wastewater treating apparatus according to claim 26, 27 28 or29, wherein the conductor capable of generating hypohalogenous acid,ozone or active oxygen by the electrochemical technique is a ceramicconductor or a conductor covered with the ceramic conductor.
 34. Thewastewater treating apparatus according to claim 26, 27, 28 or 29,wherein the conductor capable of generating hypohalogenous acid, ozoneor active oxygen by the electrochemical technique is a carbon-basedconductor or a conductor covered with the carbon-based conductor. 35.The wastewater treating apparatus according to claim 26, 27, 28 or 29,wherein the conductor capable of generating hypohalogenous acid, ozoneor active oxygen by the electrochemical technique is a stainless steelor a conductor covered with the stainless steel.